One of main goals in optical communication and data transmission systems is the most optimal connection between the transmission and receiving sides. In modern systems this goal is achieved by efficient management of optical signals, i.e., by transmitting/receiving signals between communicating sides along the shortest paths and with involvement of the minimal possible quantity of indispensable optical network components.
There exists a great amount of different optical transmission/receiving systems, which are in practical commercial use for optical communication. Recently, an interest arose to possibility of incorporating specific local networks into existing commercial communication systems. Such incorporation must be fulfilled without interference with the signals transmitted through the main lines and without necessity of modification of the aforementioned existing systems, as well as without degradation of their performance characteristics.
For example, let us assume that a long-haul commercial communication line is used for transmitting and receiving data between two sides with optical signals having a wavelength 1550 nm and that a local network incorporated into the main line operates with optical signals having wavelength of 1480 nm and 1310 nm. It is understood that elements of local networks e incorporated a section of the main communication trunk should not interfere with the optical signals transmitted through the main line and should not impair the quality of the main signals.
Incorporation of local networks into main lines can be carried out through the use of optical signal multiplexers/demultiplexers, signal add/drop optical modules, etc.
An example of a multiplexing/demultiplexing module suitable for the above-mentioned purpose is described in U.S. Pat. No. 6,252,719 issued on Jan. 26, 2001 to B. R. Eichenbaum. This module is schematically shown in FIG. 1 in the form of a beam splitter/combiner unit 20 for multiplexing and/or demultiplexing a plurality of optical signals shown in FIG. 1 by arrows with appropriate wavelengths. In the embodiment shown in FIG. 1 the system has three input/output signals of respective wavelengths λ1 λ2 λ3. The module consists of an optical fiber 22 for transmitting and receiving optical signals of wavelengths λ1 λ2 λ3, a collimating lens 24 located in front of the input/output end of the optical fiber 22, a splitter/combiner 26 and three receiver/transmitters 28, 30, and 32 with respective collimating lenses 34, 36, and 38 between the splitter/combiner 26 and respective receiver/transmitters 28, 30, and 32.
The splitter/combiner 26 includes a first dichroic mirror 40 having low loss transmission over a first preselected range of wavelengths including λ1 λ2 so as to substantially transmit signals having wavelengths λ1 λ2 through the first mirror 40 and along an optical signal path P of the module 20. The first dichroic mirror 40 also has high reflectance over a second preselected range of wavelengths including λ3 so as to substantially reflect from the first mirror 40 signals having wavelength λ3. A second dichroic mirror 42, disposed along the optical signal path P for receiving from the first mirror 40 the signals transmitted through the first mirror 40, has low loss transmission over a third preselected range of wavelengths including λ1 so as to substantially transmit signals having wavelength λ1 through the second dichroic mirror 42. The second dichroic mirror 42 also has high reflectance over a third preselected range of wavelengths including λ2 so as to substantially reflect from the second mirror 42 signals having wavelength λ2.
Other embodiments of the multiplexing/demultiplexing systems disclosed in the aforementioned patent are based on the same principle and differs from each other by positions of transmitters/receivers relative to mirrors.
A disadvantage of the systems described in U.S. Pat. No. 6,252,719 consists in that the beam splitter/combiners used in these systems are not suitable for use in a bidirectional mode with simultaneous transmission and reception of the signals along the same fiber. This is because all channels used in the system are designed only for a single-operation function, e.g., only for transmitting or only for receiving. In the context of the present invention, the term “channel” means an individual optical fiber used for transmission/reception of optical signals. For example, in the system of FIG. 1, two channels, e.g., those corresponding to the transmitters/receivers 28 and 30, can be used only for reception of the signals, while the channel corresponding to one of the channels can work as a receiver and two channels can work as transmitters. It is understood that the aforementioned module cannot be incorporated into a main long haul communication line, e.g., for bidirectional optical fiber communications between the central telephone exchange side and a subscriber side, which utilizes bidirectional transceivers having individual channels working in a transceiving and receiving modes simultaneously.
Examples of aforementioned bidirectional optical signal transceivers are described in U.S. Pat. No. 6,075,635 issued on Jun. 13, 2000 to T. Butrie, et al., U.S. Pat. No. 5,485,538 issued on Jan. 16, 1996 to T. Bowen, et. Al, and in U.S. patent application Ser. No. 10/107,4346 filed on Feb. 12, 2002 by Igor Gurevich, et al. and relating to optical module for high-speed bidirectional transceiver. U.S. Pat. No. 5,005,935 issued on Apr. 9, 1991 to T. Kunikane, et al. discloses a wavelength-division multiplexing optical transmission system, which transmits light of wavelengths λ1, λ2, λ3 (λ1<λ2<λ3) by way of a single optical fiber. An optical multiplexer/demultiplexer of the filter type is used which includes a parallelogram prism, a first filter formed on a side face of the parallelogram prism, and second and third filters formed on the opposite side face of the parallelogram. Bidirectional optical fiber communications between the central telephone exchange side and a subscriber side can be achieved using such optical multiplexer/demultiplexer of the filter type. However, similar to the previously criticized module, the parallelogram prism module of U.S. Pat. No. 5,005,935 also cannot be used in optical fiber communications systems, which utilize bidirectional transceivers having individual channels working in a transceiving and receiving modes simultaneously.
U.S. Pat. No. 6,167,171 issued on Dec. 26, 2000 to M. Grasis, et al. and U.S. Pat. No. 6,198,857 issued on Mar. 6, 2000 to M. Grasis, et al. both relate to optical multiplexing devices based on the use of optical prisms with filters and mirrors formed on external surfaces of the prisms.
Thus, U.S. Pat. No. 6,167,171 describes an optical multiplexing device comprising multiple wavelength division multiplexers cascaded together. A first one of the wavelength division multiplexers has a common port and other optical ports, which are optically coupled to the common port. The common port may be optically coupled to a trunk line of a system employing wavelength division multiplexing, for example, a fiber-optic telecommunication system employing 4, 8, 16 or other number of multiplexed channels. The optical ports include multiple channel ports, each of which is transparent to a corresponding wavelength sub-range and reflective of other wavelengths. The second-wavelength division multiplexer has a common port optically coupled to one of the optical ports of the first-wavelength division multiplexer. The second-wavelength division multiplexer also has multiple optical ports, which are optically coupled to its common port and include multiple wavelength-selective channel ports. A waveguide, such as a fiber-optic line, can optically connect the common port of the second-wavelength division multiplexer to an optical port of the first-wavelength division multiplexer. The cascaded WDMs (wavelength division multiplexers) each may be optically coupled to the output of a passive coupler and a housing may be provided defining an enclosed space in which the optical multiplexing device is mounted. Optionally, additional WDMs may be cascaded with the first two WDMs in a parallel or branched formation, an in-line formation or some combination. Preferably, the channels are interleaved, such that they are removed from the multiplexed signal in certain non-sequential order. The optical multiplexing device also may employ compound interleaving wherein adjacent channels are multiplexed by different ones of the cascaded WDMs. The optical multiplexing devices can operate to add signals, remove signals or a combination of both.
In its form as described and shown in the specification of aforementioned U.S. Pat. No. 6,167,171, the module disclosed in this patent cannot be used in conjunction with an optical fiber communications system that utilizes bidirectional transceivers with individual channels working in a transceiving and receiving modes simultaneously.
The second patent, i.e., U.S. Pat. No. 6,198,857, also relates to an optical multiplexing device for multiplexing optical signals, for example, for a fiber-optic telecommunication system employing wavelength division multiplexing. This device is an add/drop type device, which has a filter assembly defining a light path, preferably a multi-bounce zigzag expanded beam light path, from a common port at least to a first channel port and then a second channel port and then a pass-through port. The first channel port has a first optical filter element, for example, a multi-cavity interference filter, which is transparent to a wavelength sub-range within the wavelength range passed by the common port and the pass-through port, and substantially reflective of other wavelengths within such wavelength range. The second channel port includes a second optical filter element having light transmittance and reflectance properties substantially the same as those of the first optical filter element. The optical multiplexing device can be used to extract or drop a selected wavelength sub-range, most typically a single channel signal, from the multiplexed light, and then to inject a new signal into the multiplexed light at that same wavelength sub-range. In accordance with preferred embodiments, the optical multiplexing device serves as an add/drop filter arrangement to extract the signal of a particular channel and then immediately use the available channel by injecting a new signal at that same wavelength sub-range. The device described in this patent possesses the same disadvantages as all the previously analyzed references.